Valacyclovir is an L-valyl ester prodrug of acyclovir. Acyclovir is an acyclic analog of a natural nucleoside which has been found to have high anti-viral activity. Acyclovir is widely used in the treatment and prophylaxis of viral infections in humans, particularly infections caused by the herpes group of viruses. See Goodman and Gilman's, The Pharmacological Basis of Therapeutics 1193-1198 (9th ed. 1996).
Acyclovir is an acyclic guanine nucleoside analog that lacks a 3′-hydroxyl on the side chain. Acyclovir has the chemical name 6H-Purin-6-one, 2-amino-1,9-dihydro-9-[(2-hydroxyethoxy)methyl]. (CAS Registry No. 59277-89-3.) Acyclovir as the sodium salt is currently marketed as ZOVIRAX®. The chemical structure of acyclovir is shown as Formula I. 
Valacyclovir has the chemical name I-valine, 2-[(2-amino-1,6-dihydro-6-oxo-9H-purin-9-yl)methoxy]ethyl ester. (CAS Registry No. 124832-26-4.) Valacyclovir is currently marketed as VALTREX®. The chemical structure of valacyclovir is shown as Formula II. 
For oral administration, it is advantageous to administer valacyclovir rather than acyclovir because acyclovir is poorly absorbed from the gastrointestinal tract after oral administration in both animals and humans. In contrast, valacyclovir is rapidly absorbed from the gastrointestinal tract after oral administration. Moreover, valacyclovir is converted rapidly and virtually completely to acyclovir after oral administration in healthy adults. The conversion of valacyclovir is thought to result from first-pass intestinal and hepatic metabolism through enzymatic hydrolysis.
Acyclovir kills viruses by inhibiting viral DNA synthesis. Because acyclovir is a guanosine analog which lacks the 3′-hydroxyl on the side chain, it causes DNA chain termination during viral DNA replication. In virus infected cells, acyclovir is converted to the monophosphate derivative (acyclovir-MP) by a viral enzyme, thymidinine kinase. Acyclovir-MP is then phosphorylated to the diphosphate and triphosphate analogs by cellular enzyme. Incorporation of activated acyclovir into the primer strand during viral DNA replication, leads to chain termination, since without the 3′ hydroxyl the DNA chain can not be extended. Since uninfected cells lack the viral enzyme thymidine kinase, acyclovir is selectively activated only in cells infected with viruses that code for the appropriate kinases.
U.S. Pat. No. 4,199,574 discloses the treatment of viral infections with acyclovir.
U.S. Pat. No. 4,957,924 (the '924 patent”) discloses amino acid esters of the purine nucleoside acyclovir, pharmaceutically acceptable salts thereof and their use in the treatment of herpes virus infections. Also disclosed are pharmaceutical formulations and processes for the preparation of such compounds. Valacyclovir and its salts, including the hydrochloride salt, are among the disclosed compounds.
The '924 patent further discloses a method for the preparation of valacyclovir by condensation of CBZ-Valine and acyclovir in Dimethylform amide (DMF) with catalytic amount of 4-dimethylaminopyridine (DMAP) and Dicyclohexylcarbodiimide (DCC) as a coupling reagent. 
U.S. Pat. No. 6,107,302, incorporated herein by reference, discloses an anhydrous crystalline form of valacyclovir hydrochloride and a process of preparation.
The discovery of a new crystalline form of a pharmaceutically useful compound provides an opportunity to improve the performance characteristics of a pharmaceutical product. It enlarges the repertoire of materials that a formulation scientist has available for designing, for example, a pharmaceutical dosage form of a drug with a targeted release profile or other desired characteristic. It is clearly advantageous when this repertoire is enlarged by the discovery of new crystalline forms of a useful compound. For a general review of polymorphs and the pharmaceutical applications of polymorphs see G. M. Wall, Pharm. Manuf. 3, 33 (1986); J. K. Haleblian and W. McCrone, J. Pharm. Sci., 58, 911 (1969); and J. K. Haleblian, J. Pharm. Sci., 64, 1269 (1975), all of which are incorporated herein by reference.
The solid state physical properties of crystalline forms of a pharmaceutically useful hydrochloride can be influenced by controlling the conditions under which the hydrochloride salt is obtained in solid form. Solid state physical properties include, for example, the flowability of the milled solid. Flowability affects the ease with which the material is handled during processing into a pharmaceutical product. When particles of the powdered compound do not flow past each other easily, a formulation specialist must take that fact into account in developing a tablet or capsule formulation, which may necessitate the use of glidants such as colloidal silicon dioxide, talc, starch or tribasic calcium phosphate.
Another important solid state property of a pharmaceutical compound is its rate of dissolution in aqueous fluid. The rate of dissolution of an active ingredient in a patient's stomach fluid can have therapeutic consequences since it imposes an upper limit on the rate at which an orally-administered active ingredient can reach the patient's bloodstream. The rate of dissolution is also a consideration in formulating syrups, elixirs and other liquid medicaments. The solid state form of a compound may also affect its behavior on compaction and its storage stability.
These practical physical characteristics are influenced by the conformation and orientation of molecules in the unit cell, which defines a particular crystalline form of a substance. The crystalline form may give rise to thermal behavior different from that of the amorphous material or another crystalline form. Thermal behavior is measured in the laboratory by such techniques as capillary melting point, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and can be used to distinguish some crystalline forms from others. A particular crystalline form may also give rise to distinct spectroscopic properties that may be detectable by powder X-ray crystallography, solid state 13C NMR spectrometry and infrared spectrometry.